In the manufacture of appliances such as hot beverage makers and, more particularly, coffee brewers, it has become desirable to employ microwave energy as the source for heating the fluid used for making the beverage. Such devices require the use of a valve to control the flow of the fluid from one portion of the device to a second portion. Such a valve may be made of a plastic housing containing an actuator of metal, such as a bimetal element, which operates at selected temperature levels. Other valves may be made entirely of plastic or entirely of metal and may be thermally or manually actuated.
One particularly desirable thermally actuated valve for normally heated beverage making appliances is shown and described in U.S. Pat. Nos. 3,972,273 and 4,025,042. These are valves which are located in the bottom or base of a reservoir or other fluid container and which are adapted to be opened at a predetermined temperature when the fluid is heated. Although not shown in these patents, the upper or inlet end of the valves are usually closed by a filter which prevents particles in the fluid from interfering with the proper closure of the valve. Such filters generally comprise flat metal or plastic screens which are suitably affixed at their peripheries to the valve housing above the working parts of the valve.
It has been found that air often becomes entrapped beneath the screen, causing serious problems when microwave radiation is used as the source for heating the fluid in the reservoir. The entrapped air, in the form of bubbles, is of a different dielectric constant than the fluid and reacts differently to the microwave energy, causing localized heating in the area of the valve. This is particularly undesirable in valves which utilize a bimetal temperature-sensitive actuator which will sometimes become opened prematurely. In a coffee brewer, for example, such premature opening of the valve will expose the coffee grounds to water that is at a temperature lower than the minimum acceptable level for proper coffee brewing. Because the placement of the air bubbles within the valve assembly is subject to chance, the heating pattern is highly variable, causing erratic valve behavior.